1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer employing electrophotography, or a Multi Function Peripheral (MFP) having the functions of the copying machine, facsimile, and printer, and image forming method applied in such apparatuses.
2. Description of the Related Art
Image forming apparatuses of recent years employing electrophotography include an increasing number of those that can form color images such as color copying machines and color printers. The image forming apparatuses particularly include an increasing number of tandem image forming apparatuses each with a plurality of developing units provided with corresponding photosensitive elements. In the tandem image forming apparatus, images are formed on the photosensitive elements with corresponding single-color toners. The single-color toner images thereby formed are transferred onto an intermediate transfer member while being sequentially superimposed on one another to form a composite color image. Then, the composite color image is transferred at one time on a recording sheet.
In order to form a two-color image or a three-color image in the aforementioned image forming apparatuses, an image forming unit corresponding to a color that is not to be used may be started. This unwanted driving operation results in wasteful power consumption while unnecessarily shortening the lifetime of each parts. According to the structure disclosed in Japanese Patent Application Laid-open No. H9-138540, it is determined if an output image includes image writing in a color except black, and if it is determined that there is no image writing in a color except black, a developing and stirring device for the color except black is stopped while an output image is produced. According to the structure disclosed in Japanese Patent Application Laid-open No. 2001-343809, an image carrier driving unit and a developing driving unit are controlled such that a developing unit of an image forming unit that is not intended to be used in image formation is stopped during the image formation. Further, the structure disclosed in Japanese Patent Application Laid-open No. 2006-171233 includes a contact section by which an intermediate transfer belt and a plurality of photosensitive elements are selectively caused to come into contact with each other, and separated from each other. In this structure, a photosensitive element corresponding to an intended color and the intermediate transfer belt are caused to come into contact with each other, while a photosensitive element corresponding to an unintended color and the intermediate transfer belt are separated from each other.
In Japanese Patent Application Laid-open No. 2006-171233, a photosensitive element corresponding to an unintended color is separated from the intermediate transfer belt. This should be done by an additional driving structure, resulting in cost increase and upsizing of the apparatus. Japanese Patent Application Laid-open No. 2006-171233, as well as Japanese Patent Application Laid-open No. H9-138540 and Japanese Patent Application Laid-open No. 2001-343809, does not place consideration on how to deal with a situation where a color to be used for image formation changes from one page to another page when images are to be formed sequentially on a plurality of pages. According to Japanese Patent Application Laid-open No. H9-138540, a developing and stirring device for black as an initial color is always started; therefore, toner of black or of an initial color is always stirred even if black or the initial color is not used for image writing.
A problem to be solved by the invention is to prevent deterioration due to unnecessary stirring of toner or a developer of a color that is not to be consumed in image formation over a plurality of pages.